Some types of electronic displays require that input image data, when supplied by a standard video signal, be reformatted, re-ordered, or re-sequenced prior to display. Examples is include sequential-color displays and displays, like plasma displays, that use certain kinds of digital gray scale. The reformatting or conversion allows the display to operate in the simplest way while maintaining compatibility with legacy video standards. However, the data reformatting or conversion results in a need to pass a great deal of data to the display in a very short period of time if video image quality is to be maintained. The image data may typically have been stored in a frame buffer external to the display. Passing such large amounts of data to the display has numerous practical disadvantages. High data rates necessitate display electronic interconnection with high I/O pin counts that in turn increase display system production cost. Further, high data rates result in undesirably high display power dissipation. It would be desirable therefore to be able to display high-quality video images, even on displays that best operate on input image data in an order different than that of current video standards, without having to pass large amounts of data at high rates through the conversion or reformatting system and on to the display. These concerns about display system power consumption, interconnect size, bandwidth, and cost are heightened in many applications that use microdisplays, since the very nature of the application often stresses portability, compactness, and battery life. A “microdisplay” is a display that is magnified for viewing (whether by projection of an image larger than the microdisplay onto a more or less distant screen, or by the production of virtual image viewed with the display near to the eye), particularly when implemented on an integrated-circuit backplane utilizing semiconductor substrates or thin films.
To date, most “digital” displays (displays that vary some variation of a temporal characteristic of a digital signal driving or controlling a pixel's optical modulation or light-emitting means to achieve variation of the gray shade displayed by that pixel) have either had a very minimum amount of data storage at each pixel (for example 1 or 2 bits), or, if they utilized more storage per pixel, have still relied on data processing external to the pixel to such a degree that high bandwidth, high-power-consumption data transfer to and across the microdisplay was still required. On the other hand, many inventors and engineers have described more sophisticated hypothetical microdisplay architectures that have not yet found commercial application that rely on in-pixel circuitry so complex that the resulting pixel would be so large that a high-resolution microdisplay could be made only with a silicon backplane of prohibitive cost.
Dynamic random access memory (DRAM) has found only limited use to store image data in microdisplays. One reason for this is that DRAM registers only retain their data for a short, finite time. The amount of time varies from register to register or cell to cell due to inevitable variations in the silicon fabrication process. Cells that are unable to retain the data therein beyond some specified retention time may be considered to be defective. Because a DRAM memory requires periodic refresh and because it will typically have a significant, non-zero number of defective cells, such a memory architecture has heretofore been considered undesirable for storing of image data to be displayed.
Another difference between most digital displays and their historical antecedents is their gamma characteristic, which is the exponent of a power-law relationship between display brightness and input image value. Cathode ray tube (CRT) displays typically have a characteristic with a gamma value of 2 or a bit more. Digital displays, on the other hand, to date have typically been characterized by values of gamma (γ) essentially equal to 1. Providing a display with gamma values close to those of historical displays is important for a number of reasons. First, standard video cameras continue to have gamma values around 0.45, ensuring compatibility with the installed base of video displays. Second, legacy image and video recordings, whether analog or digital, require displays with γ≈2 for proper replay. Third, in the case of digital or quantized video signals and image representations, it turns out that a gamma characteristic with γ≈2 better matches the characteristics of human perception than does a gamma characteristic with γ≈1. It is desirable for the brightness steps in a display that result from numerically adjacent input data to have a constant perceptual spacing. Unfortunately, for displays having γ≈1, the perceived brightness steps are small at the high-brightness end of the grayscale but large at the low-brightness end, which produces perceptible and objectionable contouring of brightness gradients in the dark parts of displayed scenes. For displays having γ≈2, the perceived brightness steps are much closer to equal across the gray scale, and the contouring is greatly reduced. In some commercial digital displays this undesirable characteristic has been compensated for with extra bits of data. For example, standard eight-bit input image data can be mapped to the 10-bit values of a γ≈1 gray scale that are closest to the originally desired output value. Two to four extra bits of gray-scale data per color to make 10-12 bits/color is generally thought to provide an image on a display having a gamma characteristic of 1 that is roughly equivalent to an 8-bit/color image displayed on a display with a gamma characteristic of 2. However, the use of extra bits increases the amount of data storage registers needed to make a frame buffer, and it increases the needed bandwidth to transport the image data onto the microdisplay.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.